Household and business telephone sets require at least a pair of wires leading from the set back to the central telephone facilities, with one wire operating the ringer of the telephone set and the other wire being the communications wire. It is important to make sure that the wires of the telephone set are properly connected to the wires of the cable leading back to the communications center. Because there are so many telephone sets for a typical telephone system, expedient and accurate connection of the sets to the wires of the system is extremely important.
Historically the communications industry color coded the insulation of the wires in a cable in order to make it simpler for the installer to install various telephone plug receptacle assemblies, known as “phone plates” or “jacks.” The installer uses the color code of the insulation of the wires of the cable to determine the positions where each wire of the cable is to be connected to the contacts of the phone plate.
An example of this is the older type cable commonly called “station wire” that has four wires in a cable. The insulation for each of the four wires is colored differently: green, red, black and yellow. The four conductor station wire was used in most if not all residential applications. The phone plates having four sockets for receiving the four pins/prongs of a plug also had those same four colors of insulation on the short wires leading from the sockets that received the pins of the plugs to the screw heads of the wire connectors or junctions. To install the phone plate correctly you simply attached the wires of the cable to the screws of the phone plates with the color of each cable wire matching the color of the wire of the screw head. The green cable wire is connected to the screw having a green wire connected thereto, the red cable wire is connected to the screw having a red wire connected thereto, etc. The installation process was simple and self explanatory. Attach the same color of cable wire to the same color of the phone plate wire and the phones in the home worked. Phone plates that are color coded for station wire are still in common use, even though they are not currently being installed.
Later it became necessary to accommodate more complex communication devices, i.e. the “fax,” and the more complicated and sophisticated 6-wire systems were created and installed using the same four colors green, red, black, and yellow, and the two added wires were colored white and blue. The keystone jack and plug were developed and provided a smaller and more stable connection between the cable wires and the phone set wires and provided more spaces for additional wires. The six pin phone plate that this cable wire connected to had the appropriate colored wire going from the pins to the screws. Again installation was self explanatory. Match the colors of the cable wires to same colors of the phone plate wires and the six pin plate worked.
Next the 8-Wire system was developed which added wires having brown and white insulation as a pair. As long as the color of the insulation of the wires coming from the cable in the wall matched the color of the insulation of the wires of the phone plates to which they were connected, there was little chance of mismatching. That is the simplicity of the installation system.
An example of the system using this older system of wiring is as follows. This table addresses installation of three devices: An 8 pin/screw phone plate, a 6 pin/screw phone plate and a 4 pin/screw phone plate. We are mostly concerned with the 4 and 6 pin application.
TELEPHONEISDNETHERNET4-WIRE CABLESCREW*8-WIRE CABLE6-WIRE CABLEStation wire#1, #2Green / RedGreen / RedGreen / Red#3, #4Black / YellowBlack / YellowBlack / Yellow#5, #6White / BlueWhite / Blue#7, #8Orange / Brown——*The screw had the same color of wire attached to it coming from the pin as the wire that the diagram suggest be attached to the screw. 
Later, the industry changed the cable wire that is installed so that in some installations the color of the insulation on the wires of the cable no longer matched the colors of the insulation of the wires of the old station wire phone plates or the colors of the later developed phone plates as shown in the above table. Therefore, the installers no longer have the convenience of simply matching colors to achieve proper connections of wires of newer cables to the older phone plates in all situations. In order to properly install the newer telephone cable to the older phone plates that are color coded for station wire the installer must know how to match different colored wires together. This results in improper installation of wires in many instances.
The industry changed from installing four, six and eight conductor station wire in the colors identified above to category 3 and category 5 cable wire in the residential application. The categories 3 and 5 cables have a different color code so that the wire colors do not match the colors of the short wires of the phone plates. This transition from station wire began around 1997 and still continues. However, station wire is not totally obsolete. Most new homes are wired with category 5 cable today, but there are many older homes that are wired with station wire.
Category 3 cable is available in two, three and four pairs of wires. A pair is two individual wires each bearing color coded insulation and twisted around the other wire of the pair. The pairs, either 2, 3, or 4 pairs, are then collected together and covered with a overall jacket or sheath to form a cable. A three pair cable has six wires. A two pair cable has four wires. A four pair cable has eight wires.
For example, a three pair category three cable has the following individual wire colors: pair one—blue, and white with blue stripe; pair two—orange, and white with orange stripe; and pair three—green, and white with green stripe. The solid colored wires usually are paired with the white wire that bears a stripe that is the same color as the solid color wire. The following table outlines the different colors of wires in a cable.
ISDNETHERNETTELEPHONE(Category 5(Category 3(StationCable)Cable)Wire)8-WIRE CABLE/6-WIRE CABLE/4-WIRE CABLE/4 Pair Wire3 Pair Wire2 Pair WirePair 1BlueBlueGreen/RedWhite/BlueWhite/BluePair 2OrangeOrangeBlack/YellowWhite/OrangeWhite/OrangePair 3GreenGreenBlack/YellowWhite/GreenWhite/GreenPair 4Brown——White/Brown
The problem is that the phone plates commonly used and available for residential construction have the old station wire colors attached to the pins leading to the screws but the cable that is installed throughout the house is likely to be the newer category 3 or category 5 cable which has different colored individual wires than station wire of the phone plate. When connecting phone plates to category 3 and category 5 cable, the installer can no longer simply attach the cable wires to matching colors of the wires of the phone plate. This requires the installers to develop their own installation procedures when connecting the category 3 or category 5 cable to the phone plate.
In an example of connecting phone plates with a four pin/screw phone plate to category 5 cable, the installer could connect the blue wire to the screw with the green wire going to the pin , then connect the white wire with blue stripe wire to the screw with the red wire going to the pin, then connect the orange wire to the screw with the black wire going to the pin, then connect the white with orange stripe wire to the screw with the yellow wire going to the pin. The remaining two pairs of the category 5 cable would not be connected in this application. As long as this color combination was used throughout the house the phone system would work.
The problem is that there may be several different installers working on the house at different times, all with different levels of experience. If any of the installers mistakenly connect a phone plate differently than the previous example that phone plate will not work. Looking back at the previous example, if an installer connected the blue wire to the screw with the black wire going to the pin and the rest of the house was wired as described above, that phone plate would not work. When the home owners moved in they likely would call the installer and the installer would have to return to the job and reconnect that particular phone plate correctly. This costs money and time for the installer. This happens virtually all the time in the industry and is a major complaint of installation contractors. The installers usually do not like installing the phone systems because it is too confusing.
One solution is the “Keystone” system that uses the idea of a plug which when wired correctly will simply plug into the phone plate and the devices may be run off of the correctly wired system. This is accomplished by using a device that has a series of slots on the back that match the color coding of the new category wires. By using a device to force each wire into it's proper slot, one can wire the Keystone device to work properly. The problem is that not all houses are wired with the costly Keystone devices or the even more costly switch box or boxes.
The Keystone devices are 8 pin devices which aid the user to run multiple tasks from one phone plate. Not all phone plates need to be that sophisticated or costly. Therefore, if the end user wishes to use a computer, a fax machine and a telephone at the same time, he or she can do so with this device wired properly. For example a single extension phone in another room does not need the Keystone device or eight pin phone plate. This means that most contractors in a residential setting use the standard phone plates as described above.